Polyolefin resins, such as homopolymers and copolymers of polyalkylenes (e.g., polyethylene, inexpensive as compared to other engineering resins, and are employed in a number of end use applications in fairly large quantities. However, polyolefin resins are generally quite flexible and have somewhat limited mechanical properties. For example, polyolefin resins typically have a low heat deformation temperature which causes the resin to shrink or deform during molding operations and/or during exposure to high temperature environments. As such, polyolefin resins are not typically used to produce components for electronic or electrical appliances, automobile parts and parts of other apparatus.
Reinforcement agents and fillers, such as glass fibers, have been blended with polyolefin resins as a means to improve physical properties so as to off-set the disadvantages noted above. However, even though some noticeable property improvements have been obtained, the inherent physical properties of polyolefin resins still make them unsuitable for several end use applications. That is, even though the mechanical properties and/or heat deformation temperatures of polyolefin resins are improved somewhat by the incorporation of reinforcement agents, further significant property improvements are still desired.
In this connection, a conventional fiber-reinforced polyolefin resin typically exhibits enhanced anisotropy during mold shrinkage which results in deformed molded articles. As a consequence, it is difficult to produce a molded article from such conventional fiber-reinforced polyolefin compositions which exhibits and/or retains precise dimensional accuracy and shape. Thus, conventional fiber-reinforced polyolefin resins are unsuitable to form relatively large-sized molded articles or molded articles which require close dimensional and/or shape tolerances, even though such resins remain attractive for such purposes due to their relative low cost.
It has also been proposed to incorporate other thermoplastic resins as additives in polyolefin base resins so as to improve the mechanical properties of polyolefin resins. However, since polyolefin resins are generally incompatible with other thermoplastic resins, particularly with other thermoplastic engineering resins that could impart the desired property enhancements, the added thermoplastic resin is typically inhomogeneously dispersed within the polyolefin base resin (e.g., in bulk or particle forms). Thus, the incorporation of other thermoplastic engineering resins in polyolefin base resins has generally not been successful since the resin incompatibility results in surface peeling as well as insufficient physical and/or mechanical properties. As a result, these conventional resins are not a practical solution to imparting property enhancements to polyolefin resins.
A need therefore exists for polyolefin resins that have enhanced mechanical and/or deformation resistance properties so that the end use applications for polyolefins can be expanded by providing relatively lower cost molded components and parts, as compared to those conventional engineering resins now being used to form molded components for such end use applications. It is towards fulfilling such a need that the present invention is directed.
Broadly, the present invention is directed to filled (i.e., having a filler material) polyolefin resin compositions which additionally contain a liquid crystal polyester resin. The resin compositions of this invention are homogenous melt-blends and exhibit remarkable mechanical property improvements, particularly in terms of dimensional stability (e.g., resistance to heat and/or mold shrinkage).
More specifically, the compositions of the present invention are prepared by melt-blending meaningful amounts of a filler material and a specific liquid-crystal polyester resin (i.e., a polyester having an anisotropic melt-phase) with a polyolefin base so that the resulting compositions exhibit improved physical properties, particularly heat and deformation resistance properties. Thus, the filler and the liquid-crystal polyester synergistically cooperate to impart exceptional dimensional accuracy to molded articles formed of the polyolefin compositions of this invention.
Further aspects and advantages of this invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments which follow.